Optoelectronic components on an organic basis, for example an organic light emitting diode (OLED), are being increasingly widely used in general lighting.
An OLED includes an anode and a cathode with an organic functional layer structure therebetween. The organic functional layer structure may include one or a plurality of emitter layer(s) in which electromagnetic radiation is generated, one or a plurality of charge generating layer structure(s) each composed of two or more charge generating layers (CGL) for charge generation, one or a plurality of hole injection layer(s), one or a plurality of electron injection layer(s), and one or a plurality of electron blocking layer(s), also designated as hole transport layer(s) (HTL), and one or a plurality of hole blocking layer(s), also designated as electron transport layer(s) (ETL), in order to direct the current flow.
The luminance of an OLED is limited, inter alia, by the maximum current density that can flow through the diode. In order to increase the luminance of an OLED, it is known to combine one or a plurality of OLEDs one on top of another in series—so-called stacked OLED or a tandem OLED.
An OLED can age by the influence of harmful environmental influences and/or the diffusion of organic constituents. As a result, the optoelectronic properties of the OLED can vary in the course of operation. During the aging of an OLED, for example, a gradual decrease in luminance and increase in the voltage drop across the OLED can take place. In other words: the efficiency of a conventional OLED decreases during regular operation—illustrated in FIG. 10A and FIG. 10B.
FIG. 10A illustrates a measured voltage drop 1002 and a measured, normalized luminance 1006 as a function of the normalized operating duration 1004 of a conventional OLED. The luminance 1006 is normalized to the luminance of an unused OLED, i.e. at 0% operating duration 1004. The operating duration 1004 is normalized to the time at which the luminance 1006 has fallen to 70% of the original luminance (at 0% operating duration). Furthermore, the lifetime of an OLED can be limited by a change in the voltage drop across the OLED, a change in the uniformity or homogeneity of the luminous area and/or a shift in the color locus.
FIG. 10B illustrates the luminous fields 1010, 1020, 1030 of a conventional OLED. The initially homogeneous luminous image—illustrated in 1010 in FIGS. 10A and 10B—of a conventional OLED becomes only slightly inhomogeneous during the gradual aging on account of the slight current and temperature inhomogeneities during operation.
During the production of an OLED, however, particles 1008 can be included in the layers of the OLED. On account of these particle inclusions 1008, a failure of the OLED can occur during operation, said failure being manifested as a short circuit (short). Almost the entire current can flow away via the included particles 1008—illustrated as a dark spot 1008 in 1020 in FIG. 10B. As a result, the OLED can greatly heat up locally around the short circuit, as a result of which breaking (cracking), melting and/or further degradation of the component can occur. As a result, an abrupt failure of the OLED can occur, as a result of which the operating voltage falls toward zero, illustrated in 1030 in FIGS. 10A and 10B. As is illustrated in 1020 in FIG. 10A, no unambiguous indication of the developing short circuit can be discerned in the voltage drop 1002 and the luminance 1006. In the luminous image, by contrast, a dark spot is clearly formed around the particles 1008, which dark spot can increase further in size and can ultimately lead to the abrupt failure 1030 of the OLED.
Adequate countermeasures against particle inclusions that can limit the lifetime of a conventional OLED as a result of a spontaneous failure have not been available heretofore. Following preliminary tests of an OLED, for example direct or indirect methods for particle screening, for example optical microscopy or thermal measurements, a residual uncertainty with regard to a particle inclusion can nevertheless remain.
Furthermore, in conventional methods, OLEDs are operated with simple driver circuits which enable specific brightnesses to be set. These circuits supply the required electrical power for the operation of the OLED without taking account of changes in the optoelectronic properties in the OLED. In conventional methods, the luminance or the temperature of an OLED is measured and communicated for feedback to the driver circuit in order to compensate for the gradual light aging. In a further conventional method, the gradual decrease in luminance is measured by means of an external photodetector.